The present invention relates to a soft stainless steel sheet, which can be formed to an objective shape with high dimensional accuracy without occurrence of cracking even by severe or multi-stage deep drawing or cold-forging.
Application of a stainless steel excellent in corrosion resistance has been extended to various fields dealing with the deterioration of the environment. For instance, a member of a hydraulic pump, which is usually exposed to a humid atmosphere, is manufactured by shearing a stainless steel sheet 1 to a predetermined size, drawing and punching the sheared sheet 1, piercing the punched sheet 1, stretch flanging forming the pierced sheet 1 so as to expand a pierced part 2 to an expanded edge 3, as shown in FIG. 1.
Austenitic stainless steel such as SUS304 is material much superior in workability to ferritic stainless steel. But, when the austenitic stainless steel is plastically deformed to an objective shape by severe working as shown in FIG. 1, fine cracks often occur especially at the expanded edge 3.
Although the inventors investigated and researched for working conditions which enables formation of an austenitic stainless steel sheet to an objective shape without fine cracks, cracking was not completely suppressed by mere control of working conditions. Then, the inventors investigated effects of materials on occurrence of fine cracks, and reached the conclusion that cracking is assumed to be caused by the following mechanism:
When a product manufactured by working an austenitic stainless steel sheet is observed, strain-induced martensite is often detected. Generation of strain-induced martensite is distinct at a heavily deformed part such as an expanded edge 3. Such the strain-induced martensite makes a stainless steel sheet 1 harder.
When such a heavily deformed part is further worked (expanded), a work stress concentrates at boundaries of the strain-induced martensite due to difference in deformation resistance between austenite grains and the strain-induced martensite. Concentration of a work stress causes occurrence of microcracks. Microcracks are developed by distortion introduced during working and observed as fine cracks.
Fine cracks significantly degrades a commercial value of a product, but also causes troubles on the succeeding steps. It is also difficult to install such a defective member in a hydraulic pump. Furthermore, fine cracks acts as starting points of corrosion, so that a life time of a hydraulic pump is shortened.
Fine cracks are also detected in a product which is manufactured by cold-forging a stainless steel sheet to an objective shape. Moreover, demands for improvement on properties of stainless steel including longevity of forging dies is getting stronger and stronger in correspondence with adoption of severe forging conditions.
The present invention aims at provision of a soft austenitic stainless steel sheet, which is formed to an objective shape without any cracking even by severe or multi-stage deep drawing, cold forging and also has superior corrosion resistance.
A soft austenitic stainless steel sheet newly proposed by the present invention has an austenite-stability index Md30, which is defined by the formula (1), adjusted in a range of xe2x88x92120 to xe2x88x9210, a stacking fault formability index SFI, which is defined by the formula (2), adjusted at a value not less than 30 (preferably 35) and Cu concentration of precipitates not more than 1.0 mass % so as to maintain Cu content dissolved in a matrix at 1.0-4.0 mass %.
Md30(xc2x0 C.)=551xe2x88x92462(C+N)xe2x88x929.2Sixe2x88x928.1Mnxe2x88x9229(Ni+Cu)xe2x88x9213.7Crxe2x88x9218.5Mo xe2x80x83xe2x80x83(1) 
SFI(mJ/m2)=2.2Ni+6Cuxe2x88x921.1Crxe2x88x9213Sixe2x88x921.2Mn+32 xe2x80x83xe2x80x83(2) 
Not less than 70 mass % of nonmetallic inclusions dispersed in a matrix are preferably composed of MnOxe2x80x94SiO2xe2x80x94Al2O3 containing not less than 15 mass % of SiO2 and not more than 40 mass % of Al2O3, in order to improve workability. Furthermore, a work-hardening exponent n defined by an inclination of a true stress-true strain curve detected by a tensile test and elongation El detected by a uniaxial tensile test are preferably adjusted to 0.40-0.55 and not less than 50%, respectively, in order to manufacture a product without occurrence of any cracking even by multi-stage deep drawing.
For use as a cold-forged product, the steel sheet is improved in cold-forgability by adjusting a true stress not more than 1200 MPa at a true strain of 1.0 in a true stress-true strain curve obtained by a compression test at a strain speed of 0.01/second.
The newly proposed austenitic stainless steel sheet preferably consists of up to 0.06 mass % (C+N), up to 2.0 mass % Si, up to 5 mass % Mn, 15-20 mass % Cr, 5-9 mass % Ni, 1-5 mass % Cu, up to 0.003 mass % Al and the balance being essentially Fe except inevitable impurities. The austenitic stainless steel sheet may further contain at least one of up to 0.5 mass % Ti, up to 0.5 mass % Nb, up to 0.5 mass % Zr, up to 0.5 mass % V, up to 3.0 mass % Mo, up to 0.03 mass % B, up to 0.02 mass % REM (rare earth metals) and up to 0.03 mass % Ca.